As the base polymer of hot melt adhesives, various copolymers are used such as ethylene-vinyl acetate copolymers, block copolymers of styrene and conjugated diene and their hydrogenated products, ethylene-α-olefin copolymers, polyester resins, etc. Particularly, block copolymers comprising styrene and conjugated diene have a long open time and relatively well balanced adhesion and holding power; therefore, they are preferably being used in the adhesives for various disposable sanitary goods etc. and for surface protection films. For example, an adhesive composition based on a straight-chain block copolymer such as polystyrene-polybutadiene-polystyrene or polystyrene-polyisoprene-polystyrene is disclosed in JP, B, 45-41518 (corresponding U.S. Pat. No. 3,427,269). Also JP, B, 56-49958 discloses a hot melt adhesive composition utilizing (polystyrene-polybutadiene)nX branched-chain block copolymer.
However, the adhesive compositions containing a block copolymer composed of styrene and conjugated diene have a problem in their cohesion (holding power) in high temperature conditions, which is caused by the fact that the glass transition temperature (Tg) of the polystyrene block, which is the base polymer of the block copolymer, is as low as about 100° C., and there is a strong desire for improving the cohesion (holding power) of these adhesive compositions.